In communications where retransmission(s) can be requested when a data frame is received erroneously, an Automatic Repeat reQuest (ARQ) protocol for controlling the retransmission(s) may be used.
In a communications system employing the ARQ protocol, a transmitter transmits data frames to a receiver and receives acknowledgment frames from the receiver. In the following, the communications channel between the transmitter and the receiver is referred to as primary link.
The transmitter is divided in two parts: a Protocol Control Unit (PCU) and a Channel Coding Unit (CCU). The PCU contains an implementation of the ARQ protocol used over the primary link. The CCU includes lower layer functions such as channel coding and modulation for the primary link. The services of the CCU are used by the PCU. The PCU and the CCU may be placed in different locations. In the following, the communications channel between the PCU and the CCU is referred to as the secondary link.
This kind of situation may for example exist in a cellular GPRS (General Packet Radio Service) network in the communication between a Mobile Station (MS) and a Base Transceiver Station (BTS) which is controlled by a Base Station Controller (BSC) as it is shown in FIG. 1. The functions of the CCU which is a Channel Codec Unit according to GPRS are included in the BTS and the functions of the PCU which is a Packet Control Unit according to GPRS are included in the BSC. Alternatively, in some implementations the PCU is not located in the BSC, but may be located in the BTS or the GSN (GPRS support node). In the following, it is assumed that the PCU is located in the BSC.
The ARQ protocol is used in a radio interface which forms the primary link between the mobile station MS and the GPRS network via the BTS and BSC. The secondary link is formed by an Abis interface between the BSC and the BTS.
According to a conventional communications method using ARQ, the PCU stores all the data frames transmitted from the GPRS network until they are no longer needed. The exact time when transmitted data frames are no longer needed depends on the acknowledgment frames received from the mobile station MS for the transmitted data frames and on the ARQ protocol used.
When a data frame is to be transmitted, the PCU sends it to the CCU through the Abis interface, and the CCU transmits the data frame to the mobile station MS through the radio interface. Retransmissions have to be done in exactly the same way. Thus, the same data frame may have to be sent over the Abis interface multiple times, thereby consuming communications recources.